In the chemical fiber industry (man-made fiber industry) pigments such as titanium dioxide are used primarily for delustering threads (filaments). For this application it is important that the pigment particles be fine grained, because
coarse grained and agglomerated pigment particles can settle in the spinning nozzle fittings during spinning of the threads, a fact which leads to an undesirable pressure rise in the spinning apparatus with the result among others that the spinning nozzle must be changed; PA1 coarse grained and agglomerated pigment particles lead to a shift of the reflection maximum to higher wavelengths and, consequently, to yellowish color; PA1 coarse grained and agglomerated pigment particles due to a decrease in the light diffusion lead to graying; and PA1 coarse grained and agglomerated pigment particles can have a strength-diminishing effect on the filaments. PA1 due to the predetermined settling time of 24 hours, only the coarse grained and agglomerated pigment particles with a relatively large particle diameter, for example of approximately .gtoreq.2.5 .mu.m can be completely separated; PA1 due to the rinsing step a large quantity of waste consisting of water and pigment particles accumulates; and PA1 the method can only be carried out discontinuously. PA1 with which, compared to the known method, given the same throughput quantity coarse grained and agglomerated pigment particles having a smaller particle of diameter, for example approximately .gtoreq.1.0 .mu.m can be separated out to 100%; PA1 with which no rinsing is necessary; and PA1 which can be carried out continuously. PA1 at the same throughput as the known method, the invention permits separation of coarse grained and agglomerated pigment particles having smaller particle diameters, whereby pigments with very fine grain are obtained;
The powdered pigment shipments supplied by the pigment industry to the chemical fiber industry (man-made fiber industry), however, due to the manufacturing processes consist of a mixture of coarse and fine pigment particles. Further, in the chemical fiber industry (man-made fiber industry) pigments are added before spinning to the starting materials of the threads, such as polymers, in the form of a suspension. In such a suspension, however, the fine pigment particles, primary particles, may agglomerate to form relatively large coarse secondary particles.
A suspension of a pigment shipment supplied by the pigment industry contains, therefore, coarse, agglomerated, and fine pigment particles. In view of the fact that for dulling threads as mentioned hereinbelow, only fine pigment particles can be used, the coarse and agglomerated pigment particles must be separated from the suspension.
According to prior art known within the trade but not documented by publications, coarse grained and agglomerated pigment particles are separated from a suspension containing coarse grained, agglomerated and fine grained pigment particles by sedimentation by gravity as follows: An aqueous suspension of coarse grained, agglomerated, and fine grained pigment particles, titanium dioxide particles, is introduced into a cylindrical sedimentation container having a height of 860 mm. and a diameter of 895 mm. through an inlet at the top of the container. This container consists of a cylindrical case, with a cover having an inlet opening for the pigment suspension and a bottom provided with an outlet opening for removing the suspension containing the coarse grained and agglomerated pigment particles and an outlet for carrying away the suspension containing the fine grained pigment particles. The latter outlet consists of a short nozzle either inner threads, the nozzle in its upper region, the nozzle projecting approximately 50 mm into the container, and a pipe which can be screwed into the nozzle, with which the short pipe is closed. The suspension is allowed to stand for approximately 24 hours so that the coarse grained and agglomerated pigment particles can settle into the lower region of the container, and specifically below the upper edge of the nozzle. Subsequently, the pipe which projects through the cover of the container and, consequently, can be manipulated from the exterior, is screwed out of the short nozzle, so that the suspension containing primarily fine grained pigment particles can flow out of the container downward through the short nozzle. In the lower part of the container, below the edge of the short pipe, remains the suspension containing primarily coarse grained and agglomerated pigment particles. After six batches the container is cleaned by rinsing with water.
The drawbacks of this process and the known apparatus, however, are that